In addition I've removed "domain1" from "/etc/localdomains" and added it into both "/etc/remotedomains" and "/etc/secondarymx". Exim's config has been setup accordingly in order to support the secondarymx file.

The idea here is that should my Exchange Server be unavailable for any reason, incoming emails will be stored on "mailbackup.domain1" instead. The server will then "ping" the Exchange server every few seconds/minutes/hours and, once it successfully pings the server, push all stored emails.

Currently this works a treat (thanks rikgarner).

However, I'd now like to setup the second domain so that it follows the same sort of configuration. I've setup the DNS zone for "domain2" as follows:

I've then removed "domain2" from "/etc/localdomains" but only added it into "/etc/remotedomains". I'm sticking with "mailbackup.domain1" for domain2's secondary MX record, so technically there's no need to add domain2 into the secondarymx file (please correct me if I'm wrong).

The Exim Service has been restarted.

Note that domain1's email config works no problem. However according to DNSReport.com, there's a problem when attempting to send an email to "postmaster@domain2", as follows:

mailbackup.domain1's postmaster response:
>>> RCPT TO:<postmaster@domain2>
<<< 550-test.dnsstuff.com (test.DNSreport.com) [66.36.241.109] is currently not 550-permitted to relay through this server. Perhaps you have not logged into 550-the pop/imap server in the last 30 minutes or do not have SMTP 550 Authentication turned on in your email client.

What I expect to happen here is should domain2's Exchange server be unavailable, incoming emails will be stored on mailbackup.domain1 where the server will then ping domain2's Exchange server and, as soon as it's back up, push all stored emails.

I would seriously appreciate any help on this matter as both domains are live and, should domain2's Exchange server become unavailable, all incoming emails will be bounced!